In cases where a ground installation of a single frequency landing system is remotely located from other similar installations, there is no need for the ground station to radiate at an assigned time, and the approaching aircraft has no difficulty in identifying the ground installation. However, in impacted geographic locations, where there are multiple similar landing installations located relatively closely together, it is necessary to provide means for uniquely identifying at least one such same-frequency installation to the exclusion of others in the vicinity.
In conventional landing systems, such as the conventional Instrument Landing System (ILS) and the FAA Microwave Landing System (MLS), unique identification and signal exchanges between approaching aircraft and a particular ground installation are established by uniquely assigning different frequencies out of a band of frequencies to each of the various installations, and tuning the airborne units to the frequency of the selected installation. The FAA MLS system has 200 separate frequency channels assigned for its use in the band of 5000 to 5250 MHz. The ILS system has some 40 channels in paired bands allocated to its use in the vicinity of 100 and 300 MHz. Therefore, an adequate number of separately indentifiable channels for a single frequency landing system can be inferred as being between 40 and 200 channels. In my patent 4,429,312 entitled "Independent Landing Motoring System", a different type of identification of a same-frequency landing installation is discussed in which some of the signals transmitted to the aircraft are pulse encoded to identify that installation. That system is generally satisfactory when the aircraft is in a remote area isolated from other ground stations and when the aircraft has a weather radar to interrogate the ground installation, a decoding circuit added to the radar together, and an appropriate code selector switch for station selection in the cockpit.
However, not all aircraft have weather radars to interroate the ground installation. In addition, where there are several airfields in close geographic proximity, or where there are several landing installations of this type at the same airport, the same-frequency signals from all such landing systems can arrive at the aircraft simultaneously and hence they cannot be adequately separated for unique range tracking identification and guidance generation purposes. This is basically the same problem that plagues the conventional Air Traffic Control Radar Beacon System (ATCRBS) used by the FAA for air traffic control measures; it is called "garbling". The weather radar technique of my U.S. Pat. No. 4,429,312, with associated identifying codes, is thus very suitable for use at isolated remote sites, such as offshore oil rigs, but not suitable for areas with many same-frequency landing systems in close proximity. The problem comes basically from the fact that these systems, and the airborne radars all use a common frequency. Thus, there is no way to trigger one particular installation uniquely for positive identification purposes. There is, therefore, always the risk of undesirably triggering a nearby installation, with the result that confusing responses to the aircraft from both locations will be synchronously received in that aircraft.
In addition to a method of uniquely identifying a particular same frequency ground station, a very desirable characteristic for a landing system is the capability of providing range information. Range data has at least three major uses:
1. a means for alerting the pilot of his proximity to touchdown;
2. a means for automatically reducing the gain of the landing installation as the aircraft range to touchdown diminishes in order to maintain loop stability (often referred to as "course softening"); and
3. a means for using the elevational angular data provided by the landing system to determine altitude above the runway during the approach.
In the conventional ILS system, range to touchdown is generally provided by marker beacons on the ground at established distances from touchdown. These beacons radiate vertical fan shapped-beams through which the approaching aircraft passes. The range information thus acquired in the aircraft is used for pilot alerting and for "course softening" purposes.
In FAA MLS and conventional ILS practice, an alternative and more accurate measurement of range is provided by conventional TACAN/DME interrogators which are carried by almost all aircraft. The airborne TACAN/DME equipment interrogates a DME beacon that is co-located with the MLS or ILS ground installation and receives therefrom a direct measurement of range using usual DME techniques.
For some landing applications, a very precise measurement of range is required, and for this purpose, a Precision DME (usually referred to as PDME) is employed. The PDME is similar to the conventional DME, but uses faster rise time pulses to obtain higher precision. This PDME system imposes on aircraft, which have to use it in order to obtain a required very precise measurement of range, the additional burden of having installeld on board appropriate PDME airborne equipment. Another technique for obtaining precision range in a landing system is provided by the teaching of my U.S. Pat. No. 4,429,312. Range is measured in this disclosure by having the weather radar interrogate the landing system ground installation and trigger the transmission of pulsed angular guidance signals. These pulsed replies are synchronous with the weather radar interrogations and are range tracked in a conventional manner to provide precision range in the aircraft. Range measurements of higher precision can be obtained by the use of fast rise time pulses.
Both of the above described methods for identifying ground station installations (i.e., frequency selection or pulse group encoding) require additional equipment and adjustable cockpit controls for either tuning to the frequency of the ground installation, or for selecting the decodement of the signals radiated from that ground station. In addition, measurement of range, by means of marker beacons or DME equipment, requires the installation of appropriate marker beacons or DME beacons at the landing system ground installation. The measurement of very precise range requires the addition of specialized PDME equipment, both air and ground. While the use of the weather radar to provide precision range (as taught in my U.S. Pat. No. 4,429,312) eliminates the need for added PDME equipment, not all aircraft carry a weather radar. Thus, all conventional landing systems have tended to require added airborne equipment, or cockpit controls, or both, in order to achieve unique communication with and range to a selected ground installation.
In addition to the two techniques discussed above for obtaining range (i.e., the aircraft passing over marker beacons and the measurement of the time elapsed between an aircraft's transmission of an interrogation and the aircraft's reception of a reply from a transponder located at the landing system), there is the clocked station technique. That technique may be practiced using high precision clocks and low precision clocks. For example, equipment in one participating unit, such as a ground or airborne station, transmits a signal at a known time in an established very precise clock system. Equipment in a second unit, such as an aircraft, measures the time of reception of that transmitted signal in the same established clock system; by knowing the time at which the signal was transmitted, the propagation time between the two stations and thus the distance can be computed. One known use of this clocked ranging method is the United States Air Force AN/APN-169, Station Keeping Equipment (SKE).
Another means for establishing a common clock time, is for each participant to carry low-cost clocks of nominal stability and to periodically synchronize those clocks to a common time reference. Such synchronization of low-cost clocks may be established by an initial conventional two-way ranging process that determines the ranges between participants and thereafter uses measured range data, by an exchange between participants of relative clock times. Thus, the low-cost clocks of each participant are synchronized to a clock in one selected aircraft out of all participating aircraft. This synchronization process is then repeated at periodic intervals, which intervals occur frequently enough to maintain the common time base to adequate accuracy. A variation of this method of synchronizing all clocks to a clock in a selected unit, is to synchronize all clocks to an "average value" of all the clock times that exist when the clock synchronization process is initiated. See U.S. Pat. Nos. 3,412,399 and 3,434,140 to John Chisholm
Therefore, this "local" synchronization process requires a precision ranging and a data exchange or communications system, including transmitters and receivers in each participating unit. A requirement for clock synchronization equipment, in all aircraft, is undesirable in many applications, (i.e., cost, weight and complexity).
One advantage of using a common clock system is that identity may be established by use of "time slotting". In this time slotting concept, each of the participants is assigned a specific clock time at which to radiate, which time repeats at specified intervals. For example, a specific participant, such as No. 3, might radiate on the third second of every minute. Associated with this radiation at a specific time is a subsequent time interval or time slot, during which no other participant can radiate. This use of an established clock time and an associated time slot, by the participant to which it is assigned, permits reception of that transmission by other participants to be used to establish the identity of the sender of that transmission (i.e., any transmission received during that time interval must be from the participant assigned to transmit in that time slot). This use of an assigned time slot or time period to provide a protected identity system can be viewed as being similar to the use of a distinctive frequency for identity, which frequency cannot be used by another station in a specific geographic area.
While the use of established and precise clock time, with precision of the order of a fraction of a microsecond, can provide both precision ranging and unique identity, current use of such a common and precise clock time is limited by the attendant cost and complexity of very stable clocks, such as atomic clocks, or by the cost and complexity of the synchronizing equipment (i.e., communications system, etc.) required for lower cost clocks.
Considered broadly, a landing system does not inherently require the uses of multiple different frequencies since operation at all installation sites is usually performed on a single frequency. Single-frequency operation is an advantage because, if the actual landing guidance system can always operate on the same frequency for different sites, great simplification in terms of airborne equipment complexity and cost is possible. For example, the airborne receiver can be a fixed-frequency device.
Therefore, it should be appreciated that a landing system is needed which would provide both station selection and ranging data in a fixed frequency landing system, while using only airborne equipment which is installed in an IFR (Instrument Flight Rules) aircraft. Moreover, such a system should be simple and low in cost. With the eventual addition of NAVSTAR or Global Positining System (GPS) Navigation Sets on all but the smallest aircraft, it also would be very desirable to find a means to use GPS to provide channelization (i.e., identity) and range data for single frequency landing systems. This is especially true since such fixed frequency landing systems are, inherently, lower cost and are found in more locations throughout the world. Thus, there is a need for an advanced instrument landing system.